The other day I came across this nice clean web-based tool which calculates the Gunning Fog readability score for a piece of text. Just for fun, I put a few sentences from a translation I was then doing (on VAT legislation!) into the tool. It came up with scores ranging between 23 and 29.4 depending on the sentences I chose.
Gunning Fog is not the only readability indicator – others include Flesch-Kincaid, SMOG (Simple Measure of Gobbledygook), and Coleman-Liau. A comprehensive article by Ronald & Ruth Reck describes a number of them in detail. There are also lots of simple online tools available.